1. Field of the Invention
Example embodiments generally relate to a fixing device, an image forming apparatus including the fixing device, and a fixing method using, for example, electromagnetic induction heating, implemented by a fixing device incorporated in an image forming apparatus.
2. Description of the Related Art
A related-art image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction printer having two or more of copying, printing, scanning, and facsimile functions, forms a toner image on a recording medium (e.g., a recording sheet). For example, an electrostatic latent image formed on an image carrier is made visible with toner into a toner image. The toner image is transferred from the image carrier onto a recording sheet. A fixing device applies heat and pressure to the recording sheet bearing the toner image to fix the toner image on the recording sheet by various methods. Such methods include, for example, a heating roller method, a film method, and an induction heating method.
In a fixing device using the heating roller method, a heat source (e.g., a halogen lamp) heats a heating roller. The heating roller opposes a pressing roller to form a fixing nip between the heating roller and the pressing roller so as to nip a recording sheet bearing a toner image therebetween. At the fixing nip, the heating roller and the pressing roller apply heat and pressure to the recording sheet bearing the toner image.
In a fixing device using the film method, a film having a thermal capacity smaller than a thermal capacity of the heating roller is used as a heating member for applying heat to a recording sheet bearing a toner image.
In one example of a fixing device using the induction heating method, an induction heating coil wound around a bobbin is provided inside a heating roller. When an electric current is applied to the induction heating coil, an eddy current is generated in the heating roller and the heating roller generates heat.
In the heating roller method, the heating roller is preheated so that the heating roller may be heated quickly. By contrast, in the induction heating method, the heating roller may be heated up to a desired temperature quickly, even when the heating roller is not preheated.
Another example of a fixing device using the induction heating method includes both an induction heater and a heating roller. The induction heater includes an induction heating coil to which a power source applies a high-frequency voltage. The heating roller includes a magnetic heat-generating layer that has a Curie point equivalent to a fixing temperature. When the power source applies a high-frequency voltage to the induction heater, the heat-generating layer generates heat.
Thus, for example, a temperature of a ferromagnet included in the heat-generating layer increases quickly until the temperature of the ferromagnet reaches the Curie point. When the temperature of the ferromagnet reaches the Curie point, the heat-generating layer loses its magnetism. Thus, the temperature of the ferromagnet does not exceed the Curie point and is maintained at a desired temperature. The Curie point of the ferromagnet is equivalent to the fixing temperature. Therefore, the temperature of the ferromagnet is maintained at the fixing temperature.
The advantage of such an arrangement is that the heating roller may be quickly and precisely heated to a desired temperature without a complex controller, while a surface of the heating roller provides a proper release property and heat resistance.
In order to self-control an amount of heat generation, such fixing devices using the induction heating method may include a magnetic shunt layer including a magnetic shunt alloy. The magnetic shunt layer is provided between the induction heating coil and a degaussing member. When a temperature of the magnetic shunt alloy increases to the Curie point or higher, a repelling magnetic flux generated by the degaussing member cancels an induction magnetic flux generated by the induction heating coil. For example, when the temperature of the magnetic shunt alloy is near the Curie point, a magnetic permeability of the magnetic shunt alloy sharply decreases. Accordingly, the induction magnetic flux permeates the degaussing member. The degaussing member generates a repelling magnetic flux to activate a self-temperature-control function to prevent the heating roller from being heated up to the Curie point or higher.
Currently, there is market demand for an image forming apparatus capable of providing gloss-mode imaging, in which a glossy toner image is formed. To cope with such demand, a higher Curie point may be applied to the magnetic shunt alloy so that the heating roller may melt and fix toner particles forming a toner image on a recording sheet at a higher fixing temperature. Accordingly, a higher temperature may be applied as an upper temperature limit for limiting temperature increase at both end portions of the heating roller in a direction perpendicular to a conveyance direction of the recording sheet. Consequently, when a large-size recording sheet is conveyed to the heating roller immediately after small-size recording sheets are conveyed to the heating roller, the heating roller may not apply heat of a uniform temperature uniformly to the large-size recording sheet because the small-size recording sheets contact a center portion of the heating roller and draw heat from the center portion. Therefore, a temperature of heat applied by the heating roller to both end portions on the large-size recording sheet in the direction perpendicular to the conveyance direction of the recording sheet differs from a temperature of heat applied by the heating roller to a center portion on the large-size recording sheet. As a result, a fixed toner image on the center portion on the large-size recording sheet may have a gloss level different from a gloss level of a fixed toner image on the both end portions on the large-size recording sheet.
Obviously, such a gloss level difference between the center and the periphery of the sheet is undesirable, and accordingly, there is a need for a technology to minimize or eliminate such gloss level difference.